/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param head: The first node of linked list.
     * @param n: An integer.
     * @return: Nth to last node of a singly linked list.
     */
    ListNode nthToLast(ListNode head, int n) {
        if (head == null || n < 1) {
            return null;
        }

        ListNode p1 = head;
        ListNode p2 = head;
        for (int j = 0; j < n - 1; ++j) {
            if (p2 == null) {
                return null;
            }
            p2 = p2.next;
        }
        while (p2.next != null) {
            p1 = p1.next;
            p2 = p2.next;
        }
        return p1;
    }
}